Light emitting diode with structured substrate

ABSTRACT

Embodiments of the invention include a semiconductor light emitting device. The device includes a substrate having a first surface and a second surface opposite the first surface. The device further includes a semiconductor structure disposed on the first surface of the substrate. A cavity is disposed within the substrate. The cavity extends from the second surface of the substrate. The cavity has a sloped side wall.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a § 371 application of InternationalApplication No. PCT/IB2015/050323 filed on Jan. 16, 2015 and entitled“LIGHT EMITTING DIODE WITH STRUCTURED SUBSTRATE,” which claims priorityto U.S. Provisional Application No. 61/936,362, filed Feb. 6, 2014.International Application No. PCT/IB2015/050323 and U.S. ProvisionalApplication No. 61/936,362 are incorporated herein.

FIELD OF THE INVENTION

The present invention relates to a light emitting device such as a lightemitting diode with a hollow formed in the substrate.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,silicon, III-nitride, or other suitable substrate by metal-organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), orother epitaxial techniques. The stack often includes one or more n-typelayers doped with, for example, Si, formed over the substrate, one ormore light emitting layers in an active region formed over the n-typelayer or layers, and one or more p-type layers doped with, for example,Mg, formed over the active region. Electrical contacts are formed on then- and p-type regions.

FIG. 1 illustrates a prior art device described in more detail in US2012/0012856. US 2012/0012856 describes shaping the sapphire substrateof a III-nitride light emitting diode. US 2012/0012856 describes thedevice of FIG. 1 in paragraph 45. The device of FIG. 1 is “a GaN lightemitting diode [401] comprising a sapphire substrate 404 and anepitaxial layer 402. Slopes 405 and depressions 4042 are both formed” inthe substrate. A “lower portion of the sapphire substrate 404 isenveloped in a distributed Bragg reflector 407. A layer of silver glue408 is applied below the distributed Bragg reflector 407 and on slopes405 of the sapphire substrate 404 for reflecting light from the slopes405 of the sapphire substrate 404. As we can expect, the . . . [deviceof FIG. 1] has good light extraction efficiency due to increase of sidelight beams.”

SUMMARY

It is an object of the invention to provide a device with a hollow, orcavity, formed in the substrate. Such a device may efficiently extractlight through the sidewalls of the device. The cavity may take the formof a chamber in any suitable geometric shape e.g. a prism, prismatoid ora polyhedron.

Embodiments of the invention include a semiconductor light emittingdevice. The device includes a substrate having a first surface and asecond surface opposite the first surface. The device further includes asemiconductor structure disposed on the first surface of the substrate.A cavity is disposed in the substrate. The cavity extends from thesecond surface of the substrate. The cavity has a sloped side wall.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art device with a shaped sapphire substrate.

FIG. 2 is a cross sectional view of a III-nitride device structureaccording to embodiments of the invention.

FIG. 3 is a plan view of the device illustrated in FIG. 2.

FIG. 4 is a cross sectional view of a device including a shapedsubstrate, taken along axis 28, shown in FIG. 3.

FIG. 5A is a cross sectional view of a device including a shapedsubstrate, taken along axis 29, shown in FIG. 3.

FIG. 5B is a top view of a square device.

FIG. 6 illustrates a device including a high refractive index coatingand a thermally conductive material.

FIG. 7 illustrates an encapsulated device with a gap disposed betweenthe device and the encapsulant.

DETAILED DESCRIPTION

Embodiments of the invention are directed to light emitting devices suchas LEDs that extract light primarily from the sides of the device.

Though in the examples below the semiconductor light emitting device areIII-nitride LEDs that emits blue or UV light, semiconductor lightemitting devices besides LEDs such as laser diodes and semiconductorlight emitting devices made from other materials systems such as otherIII-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, orSi-based materials may be used.

FIG. 2 illustrates the device structure of a III-nitride LED that may beused in embodiments of the present invention. Any suitable semiconductorlight emitting device structure may be used and embodiments of theinvention are not limited to the arrangement illustrated in FIG. 2. Thedevice of FIG. 2 is formed by growing a III-nitride semiconductorstructure 12 on a growth substrate 10 as is known in the art. The growthsubstrate is often sapphire but may be any suitable substrate such as,for example, a non-III-nitride material, sapphire, SiC, Si, GaN, or acomposite substrate. A surface 13 of the growth substrate on which theIII-nitride semiconductor structure 12 is grown may be patterned,roughened, or textured before growth, which may improve light extractioninto the substrate.

The semiconductor structure 12 includes a light emitting or activeregion 18 sandwiched between n- and p-type regions 16 and 20. An n-typeregion 16 may be grown first and may include multiple layers ofdifferent compositions and dopant concentration including, for example,preparation layers such as buffer layers or nucleation layers, which maybe n-type or not intentionally doped, and n- or even p-type devicelayers designed for particular optical, material, or electricalproperties desirable for the light emitting region to efficiently emitlight.

A light emitting or active region 18 is grown over the n-type region.Examples of suitable light emitting regions include a single thick orthin light emitting layer, or a multiple quantum well light emittingregion including multiple thin or thick light emitting layers separatedby barrier layers.

A p-type region 20 may be grown over the light emitting region. Like then-type region, the p-type region may include multiple layers ofdifferent composition, thickness, and dopant concentration, includinglayers that are not intentionally doped, or n-type layers.

A portion of the p-type region 20 and the active region 18 is removed toexpose a portion of the n-type region 16 on which an n-contact 22 isformed.

A current blocking layer 23 may be formed on the p-type region 20 in anarea where a p-contact is formed. Current blocking layer 23 preventscurrent from being injected in the active region directly beneath thep-contact, which prevents or reduces the amount of light generated inthis region. Light generated directly beneath the p-contact may be lostto absorption by the p-contact 21. The current blocking layer 23 may beformed from any suitable material including, for example, dielectricmaterials such as oxides of silicon, SiO₂, and nitrides of silicon.

A transparent conductive layer 24 may be formed over the currentblocking layer and the remaining surface of p-type region 20.Transparent conductive layer 24 may provide current spreading in thep-type region 20. Examples of suitable materials include transparentconductive oxides such as indium tin oxide (ITO).

A p-contact 21 is formed over optional current blocking layer 23. The n-and p-contacts 22 and 21 may be any suitable material, such as aluminum,gold, or silver. The n- and p-contacts 22 and 21 need not be the samematerial. The n- and p-contacts 22 and 21 are electrically isolated fromeach other by a gap 25 which may be filled with a dielectric such as anoxide of silicon or any other suitable material.

FIG. 3 is a plan view of the n- and p-contacts illustrated in FIG. 2.The n-contact includes an n-contact pad 22 and an n-contact arm 22A thatis narrower than the n-contact pad 22 and extends from the n-contact pad22. The p-contact includes a p-contact pad 21 and two p-contact arms 21Aand 21B that are narrower than the p-contact pad 21 and extend from thep-contact pad 21. The n-contact arm 22A interposes the p-contact arms21A and 21B in the arrangement illustrated in FIG. 3. A gap 25electrically isolates the n-contact pad 22 and n-contact arm 22A fromthe p-type region of the device. Any suitable arrangement of contactsmay be used. The invention is not limited to the arrangement illustratedin FIG. 3.

Light may be extracted through the top surface of the device throughtransparent conductive layer 24.

LED wafers are often diced into square LEDs. In some embodiments, thedevice is a shape other than square. For example, the device illustratedin FIG. 3 is rectangular. The device may be any suitable shape,including but not limited to polygonal, circular, and hexagonal. In therectangular device illustrated in FIG. 3, the short side of the devicemay be, for example, 500 μm wide in some embodiments, at least 350 μmwide in some embodiments, and no more than 650 μm wide in someembodiments. The long side of the rectangular device illustrated in FIG.3 may be, for example, at least 650 μm wide in some embodiments, no morethan 700 μm wide in some embodiments, at least 550 μm wide in someembodiments, and no more than 800 μm wide in some embodiments.

In embodiments of the invention, the substrate 10 is shaped to improvelight extraction from the sides of the device. In some embodiments, acavity, also referred to herein as a hollow, is formed in the substrate10. The cavity may take the form of a chamber in any suitable geometricshape e.g. a prism, prismatoid or a polyhedron. FIGS. 4 and 5A are crosssections illustrating one example of a cavity. FIG. 4 is a partial crosssection taken along axis 28, illustrated in FIG. 3. FIG. 5A is a partialcross section taken along axis 29, illustrated in FIG. 3. FIGS. 4 and 5Ashow the shape of growth substrate 10. The semiconductor structure 12 isincluded in simplified form for reference. The contacts 21 and 22,current blocking layer 23, and transparent conductive layer 24 areomitted for clarity.

As illustrated in FIGS. 4 and 5A, the substrate 10 is shaped to form ahollow 40 that extends from the surface of the substrate opposite thesemiconductor structure 12 toward the semiconductor structure. Thehollow 40 has a triangular cross section in the cross sectionillustrated in FIG. 4. The external sidewalls of the substrate aresubstantially vertical, as in a conventional device. The externalsidewalls are often formed when the device is diced from a wafer ofdevices, as in a conventional device.

In the cross section taken along the short side of the rectangulardevice, illustrated in FIG. 4, the sloped sidewalls 42 and 44 of thehollow 40 each form an acute angle 32 and 34 with a plane 30 that isperpendicular to the growth direction of the semiconductor structure(i.e., parallel to a major plane of the semiconductor structure 12) andparallel to the surface of substrate 10 on which the semiconductorstructure is grown. Angles 32 and 34 may be the same angle though theyneed not be. The sloped sidewalls 42 and 44 each form an acute angle 60and 62 with a plane 58 and 58A that is parallel to the growth directionof the semiconductor structure and perpendicular to the surface ofsubstrate 10 on which the semiconductor structure is grown. Angles 60and 62 may be the same angle though they need not be. In someembodiments, angles 60 and 62 are 30° or less. In some embodiments, inthe cross section illustrated in FIG. 4, in addition to sidewalls 42 and44, the hollow 40 has a wall that is parallel to plane 30 (in otherwords, the cross section is a truncated triangle, rather than thetriangle illustrated in FIG. 4). The growth substrate may have surfaces50 and 52 adjacent to each of sidewalls 42 and 44. Surfaces 50 and 52may be parallel to plane 30. Alternatively, the sidewalls 42 and 44 mayextend to the outer edges of the substrate 10, eliminating surfaces 50and 52.

In the cross section taken along the long side of the rectangular devicenear an edge of the device, illustrated in FIG. 5A, the top 35 of thetriangular hollow 40 is illustrated as a dashed line. At every pointalong the dashed line depicting top 35, the cross section extending outof the plane of FIG. 5A is the cross section illustrated in FIG. 4. Thelocation of axis 29, along which the cross section illustrated in FIG.5A is taken, is illustrated in FIG. 4.

As illustrated by ray 45 in FIG. 4, light emitted by the light emittingregion of semiconductor structure 12 toward substrate 10 may be incidenton the side walls of the hollow 40, then reflected to escape the sidesof the device.

In the plan view illustrated in FIG. 3, the device is rectangular, suchthat the cross section illustrated in FIG. 5A is substantially longerthan the cross section illustrated in FIG. 4. In embodiments where thedevice is square or nearly square, as illustrated in FIG. 5B, the crosssections taken along axes 28 and 31 are similar in length. In some suchembodiments, cross sections taken along both axes 28 and 31 may be thecross section illustrated in FIG. 4.

The shaped substrate may be formed by, for example, removing substratematerial to form hollow 40, or by selectively growing a substrate toform hollow 40. Any suitable removal technique may be used, such asetching or laser blasting. The angle of incidence during laser blastingmay be selected to form the shape illustrated in the cross sectionillustrated in FIG. 4.

The hollow comprises a significant portion of the volume of thesubstrate 10. For example, hollow 40 may be at least 50% of the totalvolume of substrate 10 (i.e., the total volume of substrate 10 being thevolume of hollow 40 plus the volume of the remaining portion ofsubstrate 10) in some embodiments, and at least 60% of the total volumeof substrate 10 in some embodiments.

The substrate 10 has a thickness 41, illustrated in FIG. 4, thethickness 41 being measured in a direction perpendicular to both thefirst (top) surface on which the semiconductor structure 12 is formedand the second (bottom) surface from which the hollow 40 extends. Thethickness 41 of substrate 10 may be at least 400 μm thick in someembodiments, at least 500 μm thick in some embodiments, and no more than1000 μm thick in some embodiments. In some embodiments where the deviceis rectangular, the thickness 41 of substrate 10 is at least 90% of thelength of the short side of the rectangle.

The deepest part 43 of the hollow 40, illustrated in FIG. 4, measured inthe same direction as substrate thickness 41, may be at least 70% of thethickness of substrate 10 in some embodiments, at least 80% of thethickness of substrate 10 in some embodiments, at least 90% of thethickness of substrate 10 in some embodiments, at least 200 μm deep insome embodiments, at least 300 μm deep in some embodiments, and no morethan 500 μm deep in some embodiments.

In some embodiments, the sloped sidewalls of the hollow 40 are coatedwith a reflective material 64, as illustrated in FIG. 4. Any suitablereflective material formed by any suitable technique may be usedincluding, for example, reflective metals such as silver, reflectivecoatings such white reflective paint, or multi-layer structures such asdistributed Bragg reflectors (DBRs). The reflective material 64 may beelectrically conductive, or electrically insulating.

FIG. 6 illustrates a device including a high refractive index coatingand a thermally conductive material.

In some embodiments, all or a portion of the hollow 40 is filled with athermally conductive material 66, as illustrated in FIG. 6. Any suitablematerial may be used including, for example, metal such as copper. Thethermally conductive material may be thermally connected to a heat sinkor other suitable external structure.

In some embodiments, a high refractive index coating 68 is formed on thesurface of the device on which the n- and p-contacts are formed, asillustrated in FIG. 6. The high refractive index coating 68 may improvelight extraction from the sides of the device by increasing totalinternal reflection at the top surface of the device, which reduces theamount of light extracted from the top of the device. The highrefractive index coating 68 may have a refractive index of at least 1.5in some embodiments, at least 1.6 in some embodiments, at least 1.8 insome embodiments, and at least 2 in some embodiments. The highrefractive index coating 68 may be any suitable material formed by anysuitable technique. Examples include SiO_(x), SiO₂, SiN, and dielectricmaterials formed by evaporation. The high refractive index coating 68may be a multi-layer structure in some embodiments.

In some embodiments, a top surface 70 of the high refractive indexcoating 68 is roughened, patterned, or textured to improve lightextraction. The roughened, patterned, or textured surface may diffractlight and increase the amount of light that is radiated out the sides ofthe device. A photonic crystal structure or a grating structureoptimized for side emission may be formed on the top surface 70 of highrefractive index coating 68. For example, the top surface 70 of highrefractive index coating 68 may be formed into 3-sided pyramids 72 in aperiodic arrangement, such as, for example, a triangular lattice,honeycomb lattice, or any other suitable periodic arrangement. Thepyramids 72 may be, for example, at least 0.5 μm tall in someembodiments, no more than 2 μm tall in some embodiments, and 1 μm tallin some embodiments. The bases of pyramids 72 may be, for example, atleast 0.5 μm wide in some embodiments, no more than 2 μm wide in someembodiments, and 1 μm wide in some embodiments.

FIG. 7 illustrates an encapsulated device. The encapsulant 80 may be anysuitable material such as, for example, epoxy, resin, glass, orsilicone. The encapsulant 80 may be formed by any suitable techniqueincluding, for example, molding or a sol-gel process. In someembodiments, the encapsulant is formed separately then disposed over thedevice 1, for example by gluing directly to the device 1 or gluing to amount 84 on which the device is disposed. In some embodiments, thedevice is disposed on a mount 84 and the encapsulant 80 and the mount 84completely surround the device to prevent contaminants from reaching thedevice.

In some embodiments, the encapsulant 80 is shaped into a lens or othersuitable optical element. For example, the encapsulant 80 may be shapedinto the dome lens illustrated in FIG. 7, a Fresnel lens, or any othersuitable shape. As illustrated in FIG. 7, the encapsulant 80 may extendover the sidewalls of the device 1. The encapsulant 80 may be wideralong the bottom of the structure than the device 1.

In some embodiments, the encapsulant 80 is in direct contact with thedevice 1. In some embodiments, as illustrated in FIG. 7, a gap 82separates device 1 and encapsulant 80. Gap 82 is often filled with airbut may be filled with any suitable material. In some embodiments,encapsulant 80 has a high index of refraction. For example, the index ofrefraction of encapsulant 80 may be greater than 1 in some embodiments,at least 1.5 in some embodiments, and at least 1.8 in some embodiments.The material filling gap 82, if gap 82 is included, may be a lowabsorption, low index of refraction material. For example, the index ofrefraction if the material filling gap 82 may be no more than 1 in someembodiments.

Embodiments of the invention may have advantages over other availabledevices from which light is extracted primarily from the sides. Theembodiments described herein may have improved extraction uniformity andreduced spottiness, as compared to currently available side-emissiondevices. The embodiments described herein may have a high extractionefficiency of light from the sides of the device. The embodimentsdescribed herein are fairly compact and cost effective, because they maybe used as illustrated, without complicated, large, and expensivesecondary optics.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Any combination of the features describedabove is within the scope of the invention. For example, featuresillustrated above may be included in other embodiments, or omitted fromother embodiments. Therefore, it is not intended that the scope of theinvention be limited to the specific embodiments illustrated anddescribed.

The invention claimed is:
 1. A semiconductor light emitting devicecomprising: a substrate having a first surface and a second surfaceopposite the first surface; a semiconductor structure disposed on thefirst surface of the substrate, the semiconductor structure comprising alight emitting region sandwiched between n- and p-type regions, ann-contact connected to the n-type region, and a p-contact connected tothe p-type region, the n- and p-contacts formed on the surface of thesemiconductor structure opposite the first surface; a hollow disposed inthe substrate, the hollow extending from the second surface of thesubstrate, the hollow having a sloped side wall; and a high refractiveindex material disposed over the n- and p-contacts, the high refractiveindex material having a refractive index of at least 1.5.
 2. Thesemiconductor light emitting device of claim 1 wherein: the substratehas a thickness in a first direction, the first direction beingperpendicular to the first and second surfaces; and a deepest part ofthe hollow measured in the first direction is at least 70% of thethickness of the substrate in the first direction.
 3. The semiconductorlight emitting device of claim 1 wherein the hollow comprises at least50% of a volume comprising the volume of the hollow plus the volume ofthe substrate.
 4. The semiconductor light emitting device of claim 1wherein: the semiconductor light emitting device is rectangular; and athickness of the substrate at a thickest point of the substrate is atleast 90% of a length of a short side of the rectangular semiconductorlight emitting device.
 5. The semiconductor light emitting device ofclaim 1 wherein an angle between the sloped side wall of the hollow anda plane perpendicular to the first surface is no more than 30°.
 6. Thesemiconductor light emitting device of claim 1 wherein the first surfaceis textured.
 7. The semiconductor light emitting device of claim 1,wherein a majority of light emitted from the light emitting region isextracted into the substrate.
 8. The semiconductor light emitting deviceof claim 1, wherein light emitted from the light emitting region isextracted out of the semiconductor light emitting device through sidesof the substrate and sides of the semiconductor structure.
 9. Thesemiconductor light emitting device of claim 1, wherein thesemiconductor light emitting device is a die having four sides at outeredges of the die, each side connecting the second surface of thesubstrate with the high refractive index material, and the hollowextends along an entire length of the die between two opposite sides ofthe die.
 10. The semiconductor light emitting device of claim 1, whereinthe sloped sidewall of the hollow is coated with a reflective material.11. The semiconductor light emitting device of claim 1, wherein thehollow is filled with a thermally conductive material.
 12. Thesemiconductor light emitting device of claim 1 wherein the hollow has atriangular cross section along a first axis.
 13. The semiconductor lightemitting device of claim 12 wherein the hollow has a triangular crosssection along a second axis perpendicular to the first axis.
 14. Thesemiconductor light emitting device of claim 13 wherein thesemiconductor light emitting device is square.
 15. The semiconductorlight emitting device of claim 1 further comprising: a transparent,conductive material disposed between the semiconductor structure and thehigh refractive index material.
 16. The semiconductor light emittingdevice of claim 15 wherein a surface of the high refractive indexmaterial opposite the transparent conductive material is textured. 17.The semiconductor light emitting device of claim 16 wherein the texturedsurface of the high refractive index material comprises a plurality of3-sided pyramids arranged in a lattice.
 18. A semiconductor lightemitting device comprising a substrate having a first surface and asecond surface opposite the first surface; a semiconductor structuredisposed on the first surface of the substrate, the semiconductorstructure comprising a light emitting region sandwiched between n- andp-type regions, an n-contact connected to the n-type region, and ap-contact connected to the p-type region; a hollow disposed in thesubstrate, the hollow extending from the second surface of thesubstrate, the hollow having a sloped side wall; a lens disposed overthe semiconductor structure and the substrate, and extending alongsidewalls of the semiconductor structure and the substrate; and a gapdisposed between the semiconductor structure and the lens.
 19. Thesemiconductor light emitting device of claim 18, wherein the gap extendsbetween the lens and the sidewalls of the semiconductor structure andthe substrate.
 20. The semiconductor light emitting device of claim 18,wherein the lens completely surrounds the semiconductor structure andthe substrate above the second surface.